In the continuation of the present description and in the subsequent claims, the expression “optical code” is used to indicate any graphic representation having the function of storing encoded information by means of suitable combinations of elements with a preset shape, for example square, rectangular or hexagonal, of a dark colour (normally black) separated by clear elements (spaces, normally white), such as barcodes, stacked codes, i.e. codes with several sequences of superimposed bars and two-dimensional codes in general, colour codes, etc, and alphanumeric characters. The term “optical code” comprises graphic representations that are detectable not only in the field of visible light but also in the wavelength field comprised between infrared and ultraviolet.
Systems for automatically acquiring optically encoded information are known from the prior art, in particular systems for acquiring said information via images. Such systems comprise a unit for optically acquiring images and a unit for electronically processing said images.
The optical acquisition unit comprises an illuminator that projects a light beam onto the support where the optically encoded information is applied, an image-acquiring device that comprises an objective lens that forms an image of the Support and a sensor, which is arranged at the plane where the image is formed, or image plane, inside said image-acquiring device; typically, the sensor is arranged on a plane that is perpendicular to the optical axis of the objective lens, but there exist cases in which the plane of the sensor can be tilted with respect to said axis by a preset angle. The image formed on the sensor is transferred to an electronic processing unit by a suitable interface circuit; the electronic processing unit processes the image signal, extracting from the image the information encoded on the support. The optical acquisition unit may also comprise optical tracking systems, such as those disclosed in EP 1 345 157 A1, for measuring the distance from the sensor to the support, such as those disclosed in EP 1 067 361 A1, for automatic adjustment of the focusing of the objective lens, such as those disclosed in EP 0 785 452 A1. In most cases, there are also one or more deflecting mirrors. These components can be housed in a common container or in distinct containers.
The deflecting mirror, or the deflecting mirrors, enable the installation of the equipment for acquiring optically encoded information to be optimised from the point of view of space requirements with respect to the support where the optically encoded information is applied and then directing the field of view of the sensor and possibly also the light beam emitted by the illuminator to the desired area.
In the known systems the sensor is an array of photosensitive, linear or matrix elements, hence the name of the linear or matrix image acquisition system; in the former case the image is acquired by successive lines, in the second simultaneously on an area. In linear acquisition systems based on a linear sensor it is requested to concentrate the light emitted by the illuminator on a stripe coinciding with the field of view of the sensor.
In applications in which such acquisition systems are used, the system is fixed to a supporting structure and the support to which the information is applied, typically a pack, an envelope, a package, etc, is moved with respect to the system, for example by a conveyor belt. The greater the transit speed of the support, the faster the acquisition by the sensor has to be, or the shorter the exposure time has to be.
In linear acquisition systems known from the state of the art the main problem is to generate an extremely thin and concentrated light stripe so as to maximise the intensity of the signal received by the sensor, reducing the exposure time and permitting the image of the support to be “frozen” even when it is moving with respect to the acquisition system at relatively high speeds. On the other hand, the thinner the light stripe, the more difficult is the operation of mutual alignment between the illuminator and sensor; even a slight misalignment between the sensor and the illuminator means causes the field of view of the sensor to fall outside the peak of intensity of the light stripe, reducing the efficiency of the system and wasting most of the light emitted. A similar effect occurs if the light stripe is rotated with respect to the line of view of the sensor.
This problem is addressed by the systems of the prior art in an unsatisfactory manner.
EP 0 571 891 B1 discloses a linear acquisition system with a linear illuminator comprising a high-power sodium-vapour lamp, emitting on a line, and an optical element consisting of an elliptic reflector, the sodium lamp being placed near a focus of said ellipse; in this manner an image of said linear source is formed in the other focus of the ellipse, corresponding approximately to the position in which there is the support containing the optical information. The inventor states that this system requires no adjustment owing to the extreme precision of the manufacture of the supports for the lamp and the reflector so that the line of view of the sensor and the light stripe always overlap. In fact, this is true because the sodium lamp forms an image that is not very concentrated but is rather extended on quite a wide stripe, being a source that extends also in the direction that is orthogonal to the linear development of the source; in this case, the normal work tolerances are sufficient to guarantee a good overlap between the light stripe and the field of view of the sensor even in the event of slight misalignment. Further, the described solution discloses an illuminator that is constructed as a single block, that extends over the entire width of the field of view of the sensor, so even a small angular misalignment of the sensor, due to the length, results in a loss of efficiency at the ends of the sensor, which may even be significant, having a peak of intensity at only one point of the sensor and possibly null intensity at the ends.
In addition, the disclosed system is not coaxial, so in fact the overlap between illumination and field of view of the sensor is obtained only in a zone near the second focus of the ellipse; the ability to illuminate and identify information on supports arranged at different distances from the sensor is greatly diminished thereby making ineffective any technique for increasing the depth of focus of the objective lens, such as, for example, the automatic focusing techniques mentioned above.
The emergence on the market of solid state light sources such as LEDs and semiconductor lasers that are very compact and suitable for being grouped in linear bars has provided the designer with a class of sources that is able to focus a much thinner stripe, if combined with a suitable optical system. Simply replacing these sources inside the previously disclosed device makes it impossible to be carried out because the mechanical tolerances alone are not able to ensure the required alignment, and the greater concentration of light would further reduce the reading depth of the system.
Some of these drawbacks, but not all these drawbacks, are solved by a system like the one shown in U.S. Pat. No. 6,978,935, in which a system is disclosed in which the illuminator is divided into two linear sections, each comprising an array of semiconductor lasers, said sections being arranged laterally to the objective lens and emitting on a plane overlapping the field of view of the sensor. The compactness of the sources enables a very thin stripe to be generated, the coplanarity with the field of view of the sensor significantly extends the reading depth of the system, but the problem of alignment is increased. In fact, an independent adjustment is provided for each laser source for the alignment so as to align the single linear section, and a further adjustment that aligns the two sections with one another and with the sensor. This is a costly and complicated system, that attempts to compensate for each orientation error of the source with rectilinear and rotation movements on different axes; as each subsequent adjustment around an axis different from the first axis influences the preceding adjustment, not necessarily in the sense of an improvement, the risk is to be forced to perform an interactive procedure in which, once the illuminator has been adjusted on an axis, it is necessary to go back and correct the calibration on the preceding axis.
The present invention intends to remedy the aforesaid drawbacks.